In a conventional float line glass-making process, glass batch materials are heated in a furnace or melter to form a glass melt. The glass melt is poured onto a bath of molten material such as tin (tin bath), where the glass melt is formed and continuously cooled to form a float glass ribbon. The float glass ribbon is then forwarded to the annealing lehr for further processing and then may be cut to form solid glass articles, such as flat glass sheets. For float glass, the glass batch often includes soda, lime and silica to form soda-lime-silica based flat glass.
Unfortunately, conventional float glass (coated or uncoated) is susceptible to damage as a result alkalis such as sodium (Na) diffusing outwardly from the glass to the surface and possibly in coatings such as low-E coatings provided on the glass. On uncoated glass, the sodium upon reaching the surface may react with water or the like to produce visible stains or smears on the glass surface. Moreover, sodium diffusion into coatings on the glass can damage the coatings thereby leading to defected coated articles such as IG (insulating glass) window units, or other types of windows.
Most flat panel displays are built on expensive glass substrates such as vertically drawn fusion borosilicate glass or polished down borosilicate glass. This type of glass has low sodium (Na and/or Na2O) content. However, the cost of the source material for borosilicate glass is much higher than that of conventional soda-lime-silica based float glass. But conventional float glass has a lower manufacturing cost, but higher sodium content. During high temperature processing typically used in display panel manufacturing (e.g., 200-600 degrees C.) and/or over the lifetime of the display, the sodium can diffuse from the float glass substrate and cause deterioration of the display performance. For example, in a-Si (amorphous silicon) TFT displays such as those used in TVs, monitors, cell phones and so forth, sodium migration into the TFT and/or display medium material can lead to significant degradation of the TFT(s) and/or display medium and therefore of display characteristics.
Another issue with conventional soda-lime-silica based float glass is a higher coefficient of thermal expansion (CTE) than borosilicate glass (8.5 vs. 3.7 ppm/° C.). In order to pattern films on display substrates, a low CTE is often desired.
Many flat panel displays use two glass substrates; for example, a TFT substrate and a color filter substrate, which both typically have to be of the same type of glass. The cost of the glass substrates in a finished display panel can be in the range of 10-20% of the overall panel costs in certain instances. On the other hand, the cost of conventional float glass may be more than an order of magnitude lower than that of borosilicate glass used in many flat panel display applications. There is a need in the art to reduce the cost of glass substrates for acceptable use in display applications, and thus the overall display cost.
It is known to treat the surface of glass with materials such as aluminum (e.g., see JP 60-176952, and WO 2004/096724 to Hessenkemper, both of which are hereby incorporated herein by reference). However, such treatments are not done in an efficient manner consistent with the float process.
In view of the above, it will be apparent that there exists a need in the art for a method of making flat glass, e.g., via either the float process or a patterned-line process, including a technique for treating the glass to make it more durable, in a more efficient manner.